Methods of making optical performs are known. For example, an optical preform may be made according to a Modified Chemical Vapor Deposition (MCVD) process, such as described in Korean Patent Publication No. KR 102004013610. This kind of deposition is performed by injecting gases into the annular space formed between an insert tube and a discharge tube. The insert tube is disposed within the discharge tube and, along its length, has a constant diameter that is smaller than that of the discharge tube. Disposed within the insert tube is a soot-scraping element that includes a bar, which rotates in the interior of the insert tube and is in contact with the insert tube's inner surface (i.e., interior diameter).
International Application No. WO 89/02419, which is hereby incorporated by reference in its entirety, discloses a device for manufacturing an optical preform using an internal vapor deposition process. A tubular element is mounted at the pump side of a substrate for the removal of solid, undeposited particles. In particular, such a device includes a screw structure that follows the inner surface of the tubular element. The screw structure includes a gas conduit provided with orifices, preferably a pipe wound in the form of a spiral.
French Publication No. FR 2,266,668 and French Publication No. FR 2,253,723, each of which is hereby incorporated by reference in its entirety, disclose methods for manufacturing optical preforms.
As will be known by those having ordinary skill in the art, during the deposition of doped or undoped glass layers in the interior of a hollow glass substrate tube (e.g., Plasma Chemical Vapor Deposition or PCVD process), layers of low-quality quartz may be deposited, particularly in the region outside the path of reciprocation of the energy source moving along the length of the substrate tube (i.e., the resonator). Such low-quality quartz layers include so-called soot rings and quartz having high internal stress caused by high dopant content.
Moreover, plasma generated in the resonator can migrate slightly outside the resonator, resulting in a poorly defined plasma region, especially on the discharge side of the hollow substrate tube. Such plasma regions lead to inferior glass deposition at both reversal points, but particularly near the discharge side of the substrate tube. This gives rise to problems, such as preform fracture.
There is a need for an improved device for manufacturing an optical preform using an internal vapor deposition process that eliminates one or more of these aforementioned problems.